Fix CI errors.

Author: Oleh Kozynets

pandas/io/parsers.py

@@ -613,7 +613,8 @@ def read_csv(
     # the comparison to dialect values by checking if default values
     # for BOTH "delimiter" and "sep" were provided.
     if dialect is not None:
-        sep_override = (delimiter is None) and (sep is lib.no_default)
+        sep_override = (delimiter is None) and (sep is lib.no_default
+                                                or sep == ",")
         kwds = dict(sep_override=sep_override)
     else:
         kwds = dict()